BEIRUT -- Syrian troops intensified shelling of rebel-held neighborhoods in central Homs Sunday, according to activists who say humanitarian conditions are growing more dire and are pressing for the evacuation of 1,000 endangered families and dozens of wounded people who can't get adequate medical care.

Homs has been under siege for a week, part of a major escalation of violence around the country that forced the 300-strong U.N. observer force in Syria to call off its patrols.

"The humanitarian situation in Homs is very difficult," said Rami Abdul-Rahman, who heads the British-based activist group Syrian Observatory for Human Rights Observatory. "It is very clear that the army wants to retake Homs."

The Observatory asked the U.N. on Saturday to intervene in the violence in Homs and evacuate more than 1,000 families, including women and children, whose lives are in danger. It also said dozens of wounded people in rebel-controlled areas cannot get medicine or doctors to treat them.

Maj. Gen. Robert Mood, the chief of the observer mission in Syria, said Saturday that intensifying clashes over the past 10 days were "posing significant risks" to the unarmed observers who were spread out across the country, and impeding their ability to carry out their mandate. The observers' decision came after weeks of escalating attacks, including reports of several mass killings that have left dozens dead. The observers have been the only working part of a peace plan brokered by international envoy Kofi Annan, which the international community sees as its only hope to stop the bloodshed. The plan called for the foreign monitors to check compliance with a cease-fire that was supposed to go into effect on April 12, but they have become the most independent witnesses to the carnage on both sides as government and rebel forces have largely ignored the truce.

The statement calling off observer patrols reinforced fears that Syria is sliding ever closer to civil war 15 months after the rebellion to oust Syrian President Bashar Assad began. Opposition groups say more than 14,000 civilians and rebels have been killed since the uprising began in March 2011. The Syrian government has been waging a fierce offensive through towns and villages nationwide for the past week, trying to pound out rebels by shelling urban areas with tanks and attack helicopters. Rebels also have attacked Syrian forces, mostly trying to burn out their tanks. Britain's Foreign Secretary William Hague said that the development only underscores the need for the international community urgently to come together to compel the regime to meet its commitments.

"The United Nations Security Council will be considering its options including for the future of the U.N. Mission to Syria in light of a briefing from Major-General Mood on Tuesday," he said in a statement.

The U.S. reiterated its call for the Assad regime to comply with the plan, "including the full implementation of a cease-fire."

The Syrian government said it had informed Mood it understood the U.N. observers' decision and blamed rebels for the escalation in fighting.

The Observatory and the Local Coordination Committees, another activist group, said the shelling in Homs killed at least one person. Activists say the city's rebel-held areas have been under intense shelling and ground attacks for a week.

The LCC and the Observatory also reported intense clashes between rebels and troops in the Damascus suburb of Mleiha. The LCC said four people from both sides were killed in the fighting.

Rebels also attacked an army checkpoint in central Hama province killing at least three soldiers, the Observatory said. Both groups also reported violence in the northern provinces of Idlib and Aleppo as well as the eastern region of Deir el-Zour and the southern province of Daraa.

The LCC said at least 20 people were killed Sunday while the Observatory put the number at 14.

Syria's state-run news agency SANA said troops battled late Saturday with infiltrators from Lebanon killing six and wounding four of them. It added that Syrian forces also foiled an infiltration attempt from Turkey into the northern province of Idlib.

Syrian authorities say that weapons are being smuggled to rebels from neighboring countries.